Bioprostheses have become important in surgery, a significant area being the use of porcine xenografts for replacing diseased heart valves. The tissue of the xenografts must be fixed so that the valves may be stored, so that they will not deteriorate upon implantation, and so that they will not be rejected by the recipients. Glutaraldehyde is recognized as the most effective fixative known at this time.
In the past it has been considered of primary importance to cause the heart valve to assume its closed position during the time the tissue becomes fixed. It was believed this was necessary to assure competence in the closing of the valve after implantation. Early efforts involved stuffing the valve with cotton to force the leaflets to coapt during fixation. This had the shortcoming of failing to impart precisely the natural configuration to the valve, as well as being slow and expensive. Later, pressure fixation came into vogue, as exemplified by U.S. Pat. No. Re. 31,035. Buffered glutaraldehyde or other fixative, under physiological pressure, was applied to the outflow end of the valve, causing it to assume a closed position and a regular contour. This proved sufficiently effective to enable porcine heart valves to become a commercial reality as off-the-shelf items. However, it became recognized that the architecture and biomechanical characteristics of the valve tissue components became altered by the pressure applied to it during fixation. This reduced the flexibility of the leaflets and the durability of the collagenous component, and hence limited the longevity of the valve. Reduced fixation pressures then were employed, some being only a small fraction of the physiologic pressure used before. In one instance, a plug was inserted into the valve in order to assist it in maintaining its shape and a closed position during fixation.
In all these approaches, however, stresses are induced into the tissue of the valve, such as by forcing the valve to assume a certain shape during fixation. The resulting stresses interfere with the natural collagen biomechanics and may detract from the ability of the valve to maintain its precise architecture during years of service. Optimum flexibility is not achieved. In fact, even to simply immerse a valve or other tissue into a bath of fixative will impose hydrostatic or hydrodynamic forces which result in a strain in the tissue, and hence a stress that is harmful to the performance of the tissue.